The preparation of polyurethane polymers by the reaction of organic diisocyanates with polyols having a molecular weight of 500 or more is well-known. The initial product of this reaction, an isocyanate terminated prepolymer, may be chain extended with a variety of chain extending agents, such as water, to produce a high molecular weight polyurethane polymer. Water was one of the first chain extenders employed in preparing polyurethane polymers. The process usually involved milling on a standard rubber mill and resulting in a dry, crumbly mass.
Water is also used as a chain extending agent employing other means. In particular, an emulsion of the isocyanate terminated prepolymer is formed and the chain extension reaction is allowed to proceed in this state. The resulting emulsified, fully chain extender polymer is known as a polyurethane latex, although it is well recognized that the reaction of the prepolymer with water results in the introduction of a urea linkage and such polymer is in fact a polyurethane-urea. An alternate procedure involves the chain extension of the emulsified prepolymer in an aqueous solution of a diamine. The preparation of these emulsions is a well accepted, art recognized procedure, and the various methods employed for preparing same differ essentially in the means used to emulsify
Some of the initial polyurethane latices were prepared by emulsifying an isocyanate terminated prepolymer in water in the presence of an emulsifying agent. See U.S. Pat. No. 2,968,575, Mallonee. In U.S. Pat. No. 3,178,310, Berger, a similar procedure was employed for making latices for coating paper. An improvement of this process resided in building into the prepolymer a tertiary amino nitrogen atom and emulsifying the resulting modified prepolymer with an aqueous solution of an organic carboxylic acid. See U.S. Pat. No. 3,264,134, Vill. An additional variation of the process is disclosed in British Pat. No. 1,078,202 wherein an emulsion is prepared by salt formation, which comprises building into the prepolymer a salt forming group and emulsifying the resulting modified prepolymer in an aqueous solution of a salt-forming agent. This latter process is a versatile one and has been used extensively in the preparation of polyurethane latices. Both the latices disclosed in these patents and the methods employed in preparing them find particular application in the present invention and the disclosures of said patents are herein incorporated by reference.
While the polyurethane latices provide tough abrasion resistant films and coatings which are chemically resistant, as well as possessing those high strength characteristics of polyurethanes, many efforts have been made to improve the strength of such films and coatings.
The present invention represents the culmination of a long series of investigations, conducted largely by the inventors, directed to improving the strength of the polyurethane latex films and coatings.
It is a primary object of the invention to provide an improved polyurethane latex composition.
It is another object of the invention to provide polyurethane films and coatings having improved strength.
Generally then, it is an object of this invention to provide an improved polyurethane latex composition which provides films and coatings of improved strength.
It is another object of the invention to provide vinyl modified polyurethane films which have less surface tackiness than unmodified polyurethane films.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.